Further analyses of the reversion process on the molecular level will be made. Multiple cycles of transformation and reversion with various viruses in revertants of several species will be attempted to understand the basic cellular controls of reversion. The isolation of the suspected human endogenous oncornavirus will be attempted. Examination will be made of the possibility of existence of hybrid eco- and xenotropic viruses. To analyze further structure-function relationships of type C virus RNA and antigens. To isolate the informational moiety responsible for cell transformation. To isolate a product of virus growth which could possibly terminate viral replication. To understand the multi-component nature of the in vivo immune response to the virus.